Until recently, the cathode ray tube ("CRT") has been the primary device for displaying information. While having sufficient display characteristics with respect to color, brightness, contrast and resolution, CRTs are relatively bulky and power inefficient. With the advent of portable laptop computers, demand has intensified for a power efficient display having a lightweight and compact design.
Liquid crystal display ("LCD") systems are one such display technology. LCDs comprise a plurality of pixels arranged in a matrix. Each pixel of the matrix comprises a light valve for allowing light to pass through the display in response to a control signal. This light is generated by a backlight illuminator.
A variety of backlight illuminator designs are known in the art. Some of these designs are described in U.S. Pat. No. 5,101,325, incorporated herein by reference. However, these known designs require a large amount of energy to operate, have limited performance characteristics, and are complex to manufacture.
Presently, alternate designs for LCD backlight illuminators predominantly attempt to provide a uniform, nearly Lambertian light source modulated spatially by color and intensity. This has been realized through the use of an LCD light valve for producing a dynamic color image.
The active area of an LCD comprises less than 25% of the device's overall area, and as a result, 75% of the light emitted by the illuminator illuminates inactive areas of the LCD. Attempts to improve the utilization of the illuminator light have frequently centered on the arrangement of lens structures between the illuminator and the LCD for the purpose of directing the illuminator light only onto the LCD's active area. Such approaches, however, have several shortcomings particularly in view of the fundamental properties of Lambertian light sources. The inherent limitation of illumination from any source may be best understood by the following mathematical construct called the etendue, U, defined by: EQU dU=dp.sub.x .multidot.dpy.multidot.dx.multidot.dy
where p.sub.x and p.sub.y are the optical direction cosines of rays in a beam of radiation, referring to rectangular coordinate axes x, y, and z. In any passive optical system, the etendue must be conserved due to thermodynamic constraints. This principle is represented by the following mathematical formula: EQU S.multidot.dU=constant
where S is a characteristic dependent on the specific optical system employed.
By way of these hereinabove mathematical expressions, there are inherent limitations as to the extent to which an optical beam may be concentrated. This realization was proved by Dr. Roland Winston in his published paper in the Journal of the Optical Society of America, volume 60, pages 245-247 (1970), herein incorporated by reference.
The limiting aspect of the concentration of optical beams may be better followed by referring to the imaging optical systems shown in FIGS. 1a and 1b. Given this arrangement, the product of the area A and the solid angle .OMEGA. of radiation. Given this arrangement, the product of the area A and the solid angle .OMEGA. results in a constant: EQU A.multidot..OMEGA.=constant
Further, in FIG. 1b, a Lambertain source is shown coupled with an active site of an LCD. Here, the Lambertain source comprises a dimension h, with h' representing the relevant dimension of the active site of the LCD. Employing these variable, the following mathematical relationship exists between the angle .theta. emerging from the source and the angle .theta.' exiting from the LCD active site: EQU h.multidot..theta.=h'.multidot..theta.'.
Thus, for example, when h'=(1/2).multidot.h, the angle .theta.' exiting from the LCD is double the angle .theta. emerging from the source. In Lambertian sources, however, the angle .theta. emerging from the source is nearly 180.degree., such that the angle .theta.' exiting from the LCD should be approximately 360.degree.. However, this cannot be physically realized. Therefore, it naturally follows that some illumination must escape within the defined optical system. As such, improving the coupling of a continuous Lamberian source by means is practically unobtainable. Conversely, all Lambertian sources have unavoidable inefficiencies when coupled with an LCD having a limited active area.
Given these limitations, there exists a demand for an LCD backlight illuminator requiring less energy for operation, improved performance characteristics, and are easier to manufacture.